We have developed a new technique for evaluating cache coherent, shared-memory computers. The Wisconsin Wind Tunnel (WWT) runs a parallel sharedmemory program on a parallel computer (CM-5) and uses execution-driven, distributed, discrete-event simulation to accurately calculate program execution time. WWT is a virtual prototype that exploits similarities between the system under design (the target) and an existing evaluation platform (the host). The host directly executes all target program instructions and memory references that hit in the target cache. WWT's shared memory uses the CM-5 memory 's error-correcting code (ECC) as valid bits for a fine-grained extension of shared virtual memory. Only memory references that miss in the target cache trap to WWT, which simulates a cache-coherence protocol. WWT correctly interleaves target machine events and calculates target program execution time. WWT runs on parallel computers with greater speed and memory capacity than uniprocessors. WWT'...

We present and analyze a general model of rollback in parallel processing, The analysis points out three possible modes where rollback may become excessive; we provide an example of each type. We identify the parameters that determme a stability, or efficiency region for the simulation. Our analysis suggests the possibility of a dangerous “phase-transition ” from stabil ity to instability y in the parameter space. In particular, a rollback algorlthm may work efficiently for a small system but become inefficient for a large system. Moreover, for a given system, it may work quickly for a while and then suddenly slow down On the positive side, we give a tunable algorlthm, Filtered Rollback, that is designed to avoid the failure modes, Under appropriate assumptions, we provide a rigorous mathematical proof that Faltered Rollback m efficient, if implemented on a reasonably efficient multiprocessor. In particular, we show that the average time r to complete the simulation of a system with N nodes and R events on a p-processor PRAM satisfies

Time management is required in simulations to ensure temporal aspects of the system under investigation are correctly reproduced by the simulation model. This paper describes the time management services that have been defined in the High Level Architecture. The need for time management services is discussed, as well as design rationales that lead to the current definition of the HLA time management services. These services are described, highlighting information that must flow between federates and the Runtime Infrastructure (RTI) software in order to efficiently implement time management algorithms. 1. Introduction The Defense Modeling and Simulation Office (DMSO), through its High Level Architecture (HLA) initiative, is addressing the continuing need for interoperability between new and existing simulations within the U. S. Department of Defense. The HLA builds upon and generalizes the results of the Distributed Interactive Simulation (DIS) effort (DIS Steering Committee 1994) and...

Complex models may have model components distributed over a network and generally require significant execution times. The field of parallel and distributed simulation has grown over the past fifteen years to accommodate the need of simulating the complex models using a distributed versus sequential method. In particular, asynchronous parallel discrete event simulation (PDES) has been widely studied, and yet we envision greater acceptance of this methodology as more readers are exposed to PDES introductions that carefully integrate real-world applications. With this in mind, we present two key methodologies (con- servative and optimistic) which have been adopted as solutions to PDES systems. We discuss PDES terminology and methodology under the umbrella of the personal communications services application.

Parallel simulation is emerging as the dominant technique for studying parallel computers. However, the interconnection networks of these machines can be modeled at many different levels of abstraction, allowing researchers to trade off accuracy and performance. In this paper, we use the Wisconsin Wind Tunnel, a parallel simulator for cache-coherent shared-memory machines, to study the trade-offs of accuracy versus performance for six different network simulation models. We evaluate these models for a variety of parallel applications, cachecoherence protocols, and topologies. We show that only the two most expensive models---which model contention at individual links---are robust in the presence of high network loads or non-uniform traffic patterns.

Synchronization is often the dominant cost in conservative parallel simulation, particularly in simulations of parallel computers, in whichlow-latency simulated communication requires frequent synchronization. This thesis presents local barriers and predictive barrier scheduling,two techniques for reducing synchronization overhead in the simulation of message-passing multicomputers. Local barriers use nearest-neighbor synchronization to reduce waiting time at synchronization points. Predictive barrier scheduling, a novel technique whichschedules synchronizations using both compile-time and runtime analysis, reduces the frequency of synchronization operations. These techniques were evaluated by comparing their performance to that of periodic global synchronization. Experiments show that local barriers improve performance by up to 24% for communication-bound applications, while predictive barrier scheduling improves performance by up to 65% for applications with long local computation phases. Because the two techniques are complementary, I advocate a combined approach. This work was done in the context of Parallel Proteus, a new parallel simulator of message-passing multicomputers.

This paper reviews issues concerning the design of adaptive protocols for parallel discrete event simulation (PDES). The need for adaptive protocols are motivated in the background of the classical synchronization problem that has driven much of the research in this field. Traditional conservative and optimistic protocols and their hybrid variants --- that form the basis of adaptive protocols --- are also discussed. Adaptive synchronization protocols are reviewed with special reference to their characteristics regarding the aspects of the simulation state that influence the adaptive decisions and the control parameters used. Finally, adaptive load management strategies and their relationship to the synchronization protocol are discussed. Keywords: Simulation, Computers, Methodology.

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We describe a new algorithm, called Filter, that limits the propagation of erroneous computations in optimistic discrete-event distributed simulations. In the proposed algorithm, each message carries a bounded amount of dependency information that describes the assumptions made in the generation of the message, and, in addition, processes keep track of straggler events that have occurred in the system. This knowledge is used by processes to \ lter " out messages that depend on a preempted state by discarding them upon receipt. We describe the algorithm and its use in conjunction with time-warp, suggest several ways of reducing its potential overhead by adjusting the extent of ltering, and point out several interesting performance tradeo s that we are currently exploring. 1

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